In recent years, for example, the widespread use of the Internet and the sudden increase in the number of in-company LANs have caused a rapid increase in data traffic. Therefore, the use of optical communication systems which can perform communications at high capacity have become widespread, not only for data traffic, but also even for access traffic. In order to perform optical communications at high capacities, optical transmission is sped up and optical wavelength division multiplexing is achieved. A light wavelength filter is available as an important component for realizing wavelength division multiplexing.
The light wavelength filter filters light of a particular wavelength, and is an important component for performing optical wavelength division multiplex communication.
A light wavelength filter shown in FIG. 12 is disclosed in “Low Drive-Power Integrated Acoustooptic Filter On X-cut Y-propagating LiNbO3,” IEEEPHOTONICS TECHNOLOGY LETTERS, Vol. 3, No. 10, 1991. In a light wavelength filter 101, an optical waveguide 103 in which Ti is diffused is formed at an X-cut Y-propagating LiNbO3 substrate 102. In order to excite a surface acoustic wave, IDTs 104 and 105 are disposed on the LiNbO3 substrate 102. In order to form a surface acoustic wave waveguide, walls 106 and 107 in which Ti is diffused are disposed, one on each side of a surface acoustic wave propagation area. Here, a light wavelength filter having a narrow band at low electrical power is formed by forming the walls 106 and 107.
A light wavelength filter shown in FIG. 13 is disclosed in “LiNbO3 Tunable Wavelength Filter Using Acoustooptic Effect” (Year 200 Commemoration of Advanced Technology Symposium, “Piezoelectric Materials and Acoustic Wave Devices,” February, 2000). In a light wavelength filter 111, an optical waveguide 113 in which Ti is diffused is formed at an X-cut Y-propagating LiNbO3 substrate 112. In addition, an IDT 114 is provided for exciting a surface wave. Further, in order to form a surface-wave waveguide, a film-addition-type guide 115 is formed. Accordingly, a light wavelength filter capable of being integrated and having a narrow band is realized.
A light wavelength filter shown in FIG. 14 is disclosed in Japanese Unexamined Patent Application Publication No. 11-84331. In a light wavelength filter 121, an optical waveguide 123 and an TDT 124 are formed at a substrate 122. By changing the birefringence of the optical waveguide 123 in a mutual action area where a surface wave excited at the IDT 124 and light guided to the optical waveguide 123 act upon each other, a birefringence distribution generated in the light wavelength filter 121 is compensated, thereby making it possible to restrict an increase in a side lobe in a frequency characteristic.
In an acoustooptic filter like a light wavelength filter, an interval between light wavelengths that are multiplexed and the number of multiplexing operations vary depending upon optical communication systems. There is a strong demand for reducing costs of, in particular, an access-type optical communications system. Accordingly, in recent years, standards of, for example, CWDM (Coarse WDM) have been proposed. CWDM is for realizing a low-cost system by increasing the interval between wavelengths that are multiplexed. Therefore, in CWDM, there is a strong demand that the light wavelength filter have a flat characteristic over a broad band.
The light wavelength filters disclosed in the aforementioned “Low Drive-Power Integrated Acoustooptic Filter On X-cut Y-propagating LiNbO3,”IEEEPHOTONICS TECHNOLOGY LETTERS, Vol. 3, No. 10, 1991, and the aforementioned “LiNbO3 Tunable Wavelength Filter Using Acoustooptic Effect” (Year 200 Commemoration of Advanced Technology Symposium, “Piezoelectric Materials and Acoustic Wave Devices,” February, 2000) both have narrow-band filter characteristics. Therefore, the light wavelength filters disclosed in the aforementioned “Low Drive-Power Integrated Acoustooptic Filter On X-cut Y-propagating LiNbO3,” IEEEPHOTONICS TECHNOLOGY LETTERS, Vol. 3, No. 10, 1991, and the aforementioned “LiNbO3 Tunable Wavelength Filter Using Acoustooptic Effect” (Year 200 Commemoration of Advanced Technology Symposium, “Piezoelectric Materials and Acoustic Wave Devices,” February, 2000) are not filters having flat wavelength transmission characteristics over a broad band, and as a result of which, they are not filters that are required in, for example, CWDM.
In the light wavelength filter disclosed in the aforementioned Japanese Unexamined Patent Application Publication No. 11-84331, a phase match condition in the mutual action area is made constant by compensating the birefringence distribution existing in the light wavelength filter as a result of changing the birefringence of the optical waveguide.
However, in order to achieve a broad-band wavelength transmission characteristic in this light wavelength filter, the mutual action area length must be short.
More specifically, a wavelength transmission characteristic (indicated by a solid line X in FIG. 15) when a phase condition in the mutual action area is constant becomes a characteristic indicated by a broken line Y shown in FIG. 15 when the band of this filter is widened by a factor of 10.
FIGS. 16 and 17 show changes in the filter band with respect to the mutual action area length and changes in a required input electrical power with respect to the mutual action area length, respectively. Here, the mutual action area length, the input electrical power, and the transmission band are standardized as 1 when the characteristic shown by the solid line X in FIG. 15 is set. As is clear from FIGS. 16 and 17, when the band of the light wavelength filter is widened by a factor of 10, the mutual action area length is reduced to 1/10of the length before widening the band, and the required input electrical power is increased by a factor of 100.
Therefore, the light wavelength filter of the related art disclosed in Japanese Unexamined Patent Application Publication No. 11-84331 cannot provide a flat wavelength transmission characteristic over a broad band and at low electrical power.